This invention relates to a method for the continuous monitoring of the coating of a filamentary dielectric material, especially a manufactured fiber, with assistants, such as spin-finish oils.
The manufacture and further processing of fibers frequently necessitates coating the fibers with assistants. Typical cases are coating with spin-finish oils, for example winding oils, immediately following the spinning of the fibers or the application of flame retardants.
The evenness or unevenness of the application of the assistants to the fibers is of decisive importance for the further processing and quality of the fibers. To optimize the coating processes and also to quality-control the ongoing production, it would therefore be desirable to have a method for the continuous monitoring of coating of the fiber material. For the purposes of the present invention, unevenness is to be understood as meaning not only a qualitative statement about the constancy of the coating along the fiber but also a quantitative statement within the meaning of the statistical definition of the unevenness U   U  =            1                        x          _                -        T              ⁢                  ∫        0        T            ⁢                        "LeftBracketingBar"                                    x              i                        -                          x              _                                "RightBracketingBar"                ⁢                  xe2x80x83                ⁢                  ⅆ          t                    
where {overscore (x)} is the mean, xi is the momentary value and T is the evaluation time for a given variable, for example the capacitance.
Instead of in terms of the unevenness U, irregularity is frequently also reported in terms of the coefficient of variation CV, which is defined as   CV  =            1              x        _              ⁢                            1          T                ⁢                              ∫            0            T                    ⁢                                                    (                                                      x                    i                                    -                                      x                    _                                                  )                            2                        ⁢                          xe2x80x83                        ⁢                          ⅆ              t                                          
The coefficient of variation takes greater account of larger deviations from the mean than of smaller deviations, owing to the squaring within the integral. Insofar the text which follows refers generally to unevenness measurements, this is to be understood as encompassing a determination of the coefficient of variation or of some other comparable statistical variable.
From U.S. Pat. No. 4,845,983 and U.S. Pat. No. 4,862,741 an apparatus is known for measuring the unevenness of textile fibers by utilizing capacitive electrical methods of measurement to provide statistical information about the quality of fiber production. The device described in the above mentioned patents is commercialized under the trade name USTER(copyright)-TESTER by Swiss company Zellweger Uster AG. It determines the unevenness of a filamentary dielectric material by passing the filament between the electrodes of a capacitive measuring element, originally, the space between the electrodes contains only air or some other gas whose dielectric constant E is virtually 1. The introduction of the filamentary dielectric material increases the capacitance of the capacitor. If, then, as the filament passes through, variations in filament thickness occur, these give rise to corresponding variations in the capacitance of the measuring capacitor. The time profile of the capacitor""s capacitance can finally be used to calculate the unevenness U or the coefficient of variation CV of the spun filament according to the abovementioned formulae.
However, this measuring principle is not suitable for determining the unevenness of fiber coatings, since, first, the thickness of the coating material is very small compared with the thickness of the fiber, so that unevennesses in the coating do not generate an analyzable signal in a capacitive measurement, especially not in a continuous on-line measurement of ongoing production. Secondly, in the case of manufactured fiber, the dielectric constants of the fiber material and the coating material are of the same order of magnitude. The substances in question are usually apolar, weakly polar or at best medium-polar substances. Accordingly, any capacitance changes detected in the measuring element are essentially dominated by the unevenness of the fibers themselves, so that it is impossible to say anything about possible unevennesses of the coating.
It is an object of the present invention to provide a method for the continuous monitoring of the coating of a filamentary dielectric material with assistants which through a simple nondestructive measurement enables an accurate determination to be made of the unevenness of the coating. The measurement shall be virtually inertialess so as to make possible even the continuous on-line monitoring of filaments produced on modern high-speed spinning machinery at a linear filament speed of 5000 m/min and more. The method of measurement shall be relatively simple, reliable and economical.
We have found that this object is achieved by a novel method for the continuous monitoring of the coating of a filamentary dielectric material with assistants, which comprises applying a solution of said assistants dissolved in a polar solvent to said filamentary material, then passing said filamentary material between the electrodes of a downline capacitive measuring element, determining the capacitance changes of said downline capacitive measuring element, and using said capacitance changes to calculate the unevenness of the layer thickness of the applied solution.
The underlying concept of this invention is accordingly that an even application of the solution is associated with an even coating of the filamentary material with assistants. It is therefore proposed according to the invention that the unevenness of the solution layer be determined immediately after the solution has been applied, without any wait until after the solvent has evaporated or been absorbed by the fiber before measuring the unevenness of the layer of assistant or assistants then present on the fiber. The invention exploits the fact that numerous assistants, although themselves apolar or only weakly polar, are soluble in polar solvents. For instance, the spin-finish oils LIMANOL ST24RN (Schill+Seilacher, Bxc3x6blingen, Germany) or FASAVIN HA (Zschimmer and Schwarz, Lahnstein, Germany) are easy to prepare and apply to polyamide fibers as 10:90 oil/water mixtures. The solution applied to the fiber accordingly has a significantly higher dielectric constant than the fiber material itself. In the proposed measurement of capacitance changes due to the passing filament, the main signal is accordingly due to the applied solution and no longer due to the unevennesses of the fiber.
The method of the invention may utilize a very wide range of combinations of fiber material, solvent and assistant, provided the solvent has a higher dielectric constant than the fiber material.
In one advantageous embodiment of the method of the invention, said determining of said capacitance changes is effected by subtracting from the capacitance measured in said downline capacitive measuring element a reference capacitance previously determined for the uncoated filamentary material.
When the differences in dielectric constant are very large between the fiber material and the solvent, it is sufficient to use a reference capacitance which is a predetermined, constant value dependent only on the filament material and the average filament thickness. However, this method is unable to compensate for capacitance changes due to diameter variations of the moving filament, since the reference capacitance is always a constant value. But these variations are negligible in the case of widely different dielectric constants, since in this case capacitance changes are substantially due to unevennessses in the polar solvent layer.
For a more accurate measurement, the reference capacitance is continuously determined by passing said filamentary material between the electrodes of an upline capacitive measuring element prior to said applying of said solution, determining the capacitance changes of said upline capacitive measuring instrument and subtracting said capacitance changes from the capacitance changes measured using said downline capacitive measuring element by correlating the measured values in such a way that measured values associated with the same filament section are subtracted from each other in each case. This is preferably accomplished through electronic buffer storage of the values measured using the upline capacitive measuring element and subsequent electronic subtraction. This is because, as a filament section passes initially the upline capacitive measuring element and the respective capacitance change is measured at a certain time t1xe2x80x2, it is possible to determine from the known filament speed v and the distance s between the upline and downline capacitive measuring elements the time difference xcex94t after which this filament section will have reached the downline capacitive measuring element (xcex94t=s/v). Consequently, the capacitance change measured at the upline capacitive measuring element at time t1 is substracted from the capacitance change measured at the downline capacitive measuring element at the time t1xe2x80x3=t1xe2x80x2+xcex94t. This method of measurement makes it possible for the individual diameter variations of the filament to be compensated. The method of measurement is therefore especially useful for such monitoring tasks where the dielectric constant difference between the solvent and the fiber material is relatively small.
For a particularly simple method of measurement constant voltage is applied to the capacitor and capacitance changes are detected by measuring charging and discharging currents. This is because the charge on a capacitor maintained at constant voltage is given by the relationship Q=CU, where Q is the charge, C is the capacitance and U is the voltage. If, then, the capacitance of the capacitor changes on account of the dielectric present between the electrodes of the capacitor changing with time, this will be associated with a change in the charge. This change in the charge can then be detected as the flow of a current.
Particularly sensitive and fast methods of measurement are made possible through RF impedance measurements where each capacitive measuring element combined with a normal inductor forms an electrical tuned circuit which is coupled to a conventional grid dip oscillator or, if instead of a valve a transistor is used for excitation, to a conventional transistor dipper.
The method of the invention is preferably used for monitoring the coating of manufactured fibers, for example of polyamides, such as nylon-6 or nylon-6,6. The polar solvent used is preferably water, since water has the very high dielectric constant of 80. However, it is also possible to use other polar solvents, for example alcohols or aqueous alcoholic solvents.
Useful assistants include a very wide range of compositions. The method is particularly preferably used for monitoring the coating of textile fibers with spin-finish oils, for example with winding oils. Typically, the winding oil will have been dissolved in water in a concentration of from 5 to 10%. The freshly spun polyamide filaments are initially moisture-free following their solidification in the quench chimney. Before being wound up, they have a water/winding oil system applied to them by means of a spin-finish application roll. The water evaporates later or is absorbed by the fiber. The spin-finish oil finally remains behind on the fiber.
We have determined that the method of the invention is particularly simple to carry out using conventional, commercially available unevenness measurement instruments such as the USTER(copyright)-TESTER already mentioned above.
The present invention accordingly also provides for the use of an apparatus for the capacitive unevenness measurement of fibers, especially an apparatus of the USTER(copyright) TESTER type, in a method for monitoring the coating of a filamentary dielectric material with assistants.